Fuel assembly with consumable poison, and a method of operating a reactor

ABSTRACT

The fuel assembly for a pressurized water reactor comprises rods distributed at the nodes of a square array and containing the fuel material. Some of the rods have a first mass concentration of gadolinium lying in the range 5% to 12%. Four to twelve additional rods (16) contain gadolinium at a concentration lying in the range 0.5% to 2%.

FIELD OF THE INVENTION

The invention relates to fuel assemblies for pressurized water nuclearreactors, and more particular by assemblies of the type comprising fuelrods distributed at the nodes of a square array, with at least most ofthe rods containing fuel material which, in most of the rods, is uraniumoxide enriched in isotope 235 and/or in plutonium; the rods aregenerally carried by a skeleton having two endpieces interconnected byguide tubes and by rod-holding grids.

BACKGROUND OF THE INVENTION

Assemblies of this type are already known in which some (generally lessthan one in ten) of the fuel rods contain a neutron poison constitutedby gadolinium at a mass concentration in the range of 5% to 12%. Thegadolinium performs two functions. Because it absorbs neutrons, itserves to reduce the initial reactivity of the core after partial ortotal reloading with new fuel assemblies. The progressive disappearanceof the gadolinium serves to compensate for the progressive using up ofthe fuel. By an appropriate distribution of fuel assemblies, containinggadolinium in the core, it is possible to achieve a more uniform radialdistribution of power, and to do this throughout an operating cycle ofthe core prior to reloading.

The use of such assemblies in which rods that contain gadolinium allhave the same concentration, does indeed have a favorable effect fromthe point of view of reducing reactivity at the beginning of each cycle,with a corresponding reduction in the concentration of boron that isrequired in the water, and with good radial neutron flux distribution.However, it is not possible to achieve satisfactory core management whenit is designed to extend the duration of each core utilization cycle toperiods of 18 months to 24 months, and/or when it is necessary to limitradial neutron leakage for better utilization of the fuel.

SUMMARY OF THE INVENTION

The present invention seeks in particular to provide a fuel assemblythat makes more favorable core management possible, in which initialreactivity is high.

The invention makes use of the observation that the initialanti-reactivity (i.e., in a new rod) provided by the gadolinium presentin the rod is not proportional to the concentration thereof, butincreases much more slowly once that concentration exceeds about 1%.

Consequently, the invention proposes an assembly of the above-definedtype in which the rods comprise, in addition to rods having a massconcentration of gadolinium lying in the range about 5% to about 10%,four to twelve rods having a gadolinium concentration in the range of0.5% to 2%, and advantageously about 1%.

The anti-reactivity of the rods having low gadolinium concentrationdecreases much more quickly, starting from the beginning of the reactorcycle, than does the reactivity of rods having a considerably greaterconcentration. Thus, the rods having a low gadolinium content compensatefor the initial high excess reactivity and provide extra margin on themoderator coefficient without thereby having an unfavorable effect onradial power distribution within a cycle.

It may be considered that the invention makes it possible to actindependently on the two functions of the gadolinium: reducing initialreactivity, and controlling radial flux distribution. With a singleconcentration, the first function can be provided in a long cycle onlywith a high number of rods having 10% to 12% gadolinium, and that woulddeteriorate control of power distribution, in particular towards the endof a cycle as the gadolinium progressively disappears.

BRIEF DESCRIPTION OF THE DRAWINGS

The above characteristics, and others, appear more clearly on readingthe following description of particular embodiments of the invention,given by way of example and with reference to the accompanying drawings,in which:

FIG. 1 is a schematic plan view showing the distribution of fuel rods ina first assembly of the invention;

FIG. 2 shows the variation of the anti-reactivity of a rod as a functionof its Gd content, in two assemblies respectively containing oxide fuelenriched in uranium 235 to 4.50% (solid line curve) and to 3.90% (dashedline curve);

FIG. 3 is a diagram showing one possible distribution for assemblies inone-fourth of a nuclear reactor core, depending on the way in which theassemblies are implemented;

FIG. 4 is similar to FIG. 1 and shows a second distribution for eightrods having high gadolinium content; and

FIGS. 5 and 6 show how the peak power factor varies over time.

DESCRIPTION OF PREFERRED EMBODIMENT

The complete structure of an assembly to which the invention applies isnot described herein since it may be conventional. FIG. 1 merely shows15×15 locations occupied by fuel rods and guide tubes such as 10 in anassembly of the kind commonly used at present. The central location 12is occupied by an instrumentation tube. The other locations are occupiedby fuel rods.

Assemblies are already known in which sixteen rods 14 represented byshading in FIG. 1, have an initial gadolinium oxide content of 10% byweight. This content is not limiting. In other reactors, it is possibleto use a lower content, e.g., in the range of 5% to 10%. It is alsopossible to use a content slightly in excess of 10%, and to go up as faras 12%. Depending on the reactor, the rods containing gadolinium containeither uranium that is enriched in U235 or in plutonium, with enrichmentbeing equal to less than that of adjacent rods, or natural uranium, oreven depleted uranium. In other cases, four to twelve rods 14, or twentyrods 14 are provided.

In prior art assemblies, all of the rods containing a consumable poisonhave the same gadolinium content. These rods are distributed uniformly.

In the assembly of the invention shown in FIG. 1, four rods 16 marked bycrosses contain a low initial content of gadolinium. These rods 16 aredisposed near the corners of the assembly, e.g., occupying the next tolast location on each diagonal, when the rods 14 are distributed in themanner described above. It is possible to choose the same uranium mediumas for the rods having 10% gadolinium.

FIG. 2 shows that the initial anti-reactivity effects provided by a rod14 and by a rod 16 are not proportional to the gadolinium contentsthereof. For example, in FIG. 2 it can be seen that for an assemblycomprising rods initially enriched to 4.50% (ratio of U 235/(U 238+U235)), the anti-reactivity provided by the 1% gadolinium is a littlegreater than 500 pcm (parts per hundred thousand), whereas theanti-reactivity provided by 8% gadolinium is no more than about 750 pcm.As a result, reducing the initial gadolinium content in a ratio of 8 to1 gives rise to a reduction in anti-reactivity in a ratio of only about1.47 to 1.

Still by way of example, FIG. 3 shows one possible distribution of fuelassemblies after one-third of the assemblies have been replaced by newassemblies. The shaded locations are occupied by assemblies that havealready been used once or twice in the reactor.

Assemblies occupying the locations that have no marking have nogadolinium. The others contain four rods containing gadolinium at thesecond concentration, i.e., about 1%. They also contain sixteen rodshaving 10% gadolinium, of the kind shown in FIG. 1, or only eight rodscontaining 10% of gadolinium at the first concentration, in thearrangement shown in FIG. 4 where elements corresponding to those of thearrangement in FIG. 1 are given the same reference numerals.

As mentioned above, the fuel of the rods containing gadolinium may havevarious compositions. Nevertheless, it generally appears preferable forrods containing gadolinium at the second concentration to adopt a levelof enrichment that is close to that of the adjacent rods having nogadolinium.

This result appears particularly on comparing the results obtained withvarious concentrations of U235 in the fuel. In particular, tests havebeen performed with assemblies having sixteen rods with 10% Gd and fourrods with 1% Gd, the number of assemblies being such that there are onehundred and twelve rods with 1% Gd in all. FIG. 5 shows the resultsobtained with 1% Gd rods and with fuel comprising uranium oxide depletedto 0.25% uranium 235, uranium oxide enriched to 2.30% uranium 235, anduranium enriched to 3.90% uranium 235 (i.e., enriched to the same extentas the adjacent un-poisoned rods). The advantage of using the samedegree of enrichment for all of the rods can be seen, in that there islittle degradation of the radial peak power factor Fxy relative tomanagement of the prior art type, and cycle duration is of the order of350 fped (full power equivalent days).

Other tests have shown that in all cases the initial concentration ofboron can be reduced considerably.

The result may be further improved by also adding four rods having 1% Gdto assemblies that contain only eight rods at 10% Gd, the total numberof rods containing 1% Gd then being one hundred and sixty for the coreas a whole. FIG. 6 shows how the radial peak factor Fxy varies as afunction of exhaustion in reference conditions and for a core having onehundred and sixty rods with 1% Gd on fuel enriched to 2.30% U 235.

In this case, compared with reference management, the difference ininitial boron concentration may be as much as -103 pcm, and there mayalso be a reduction in the moderator temperature coefficient of 1.7pcm/°C.

Numerous other embodiments are possible. The assembly may be of anyknown type, having an array in which the number of nodes varies over14×14 to 20×20. Rods containing gadolinium at the second concentrationmay be disposed in any one of numerous dispositions that are circularlysymmetrical to order 4. In particular, rods containing gadolinium at thesecond concentration may be placed in the edge row; rods may be locatedat nodes adjacent to the location of a guide tube or of aninstrumentation tube. If no central instrumentation tube is provided,then a rod containing gadolinium at the second concentration may beplaced at the center of the assembly.

We claim:
 1. A fuel assembly for a pressurized water reactor, theassembly comprising rods distributed at the nodes of a square array andcontaining a fuel material, some of said rods having a firstconcentration of gadolinium lying in the range 5% to 10% by weight,wherein four to twelve rods other than said some of the rods have asecond concentration of gadolinium lying in the range 0.5% to 2% byweight.
 2. An assembly according to claim 1, wherein the fuel of therods containing gadolinium at a second concentration is constituted byuranium oxide enriched in U 235 to an extent that is less than or equalto that of those of the rods in the fuel assembly that do not containgadolinium.
 3. An assembly according to claim 1, wherein the fuel ofrods containing gadolinium at the first concentration is constituted bynatural uranium oxide that is depleted or enriched in uranium
 235. 4. Anassembly according to claim 1, wherein the rods containing gadolinium atthe first concentration are four to twenty in number.
 5. An assemblyaccording to claim 1, wherein the rods containing gadolinium at thesecond concentration are distributed in the edge row with circularsymmetry of order
 4. 6. An assembly according to claim 1, wherein therods containing gadolinium at the second concentration are distributedwith circular symmetry of order 4 at the nodes of the array adjacent tothe nodes occupied by guide tubes or by a central instrumentation tube.7. An assembly according to of claim 1, having no centralinstrumentation tube, wherein a rod containing gadolinium at the secondconcentration is placed at the center of the assembly.
 8. An assemblyaccording to claim 1, wherein said rods having gadolinium at a secondconcentration include four rods having a gadolinium content of 1%situated at locations on diagonals on the array in the last row before aperipheral row of said array.
 9. A fuel assembly for a pressurized waterreactor, comprising a plurality of fuel rods distributed at the nodes ofa 15×15 square array, each of said rods having a sheath and containingfuel material,wherein four to twenty of said fuel rods have a firstconcentration of gadolinium lying in the range 5% to 12% by weight, andwherein four to twelve other ones of said fuel rods have a secondconcentration of gadolinium lying in the range 0.5% to 2% by weight andare regularly distributed in a peripheral row of said array.
 10. A coreof a pressurized water reactor comprising a plurality of fuel assembliesdistributed in a square array, each of said fuel assemblies comprising aplurality of fuel rods distributed at the nodes of the square array,some of said fuel assemblies containing partially burnt-up fuel andremaining ones of said fuel assemblies being new fuel assemblies;whereinfirst ones of said new fuel assemblies are devoid of gadolinium; secondones of said new fuel assemblies are located close to the periphery ofthe core and include eight fuel rods having a content of gadolinium in arange between 5 and 12% and four fuel rods having a content ofgadolinium between 0.5 and 2%, the other fuel rods being devoid ofgadolinium; and remaining ones of said new fuel assemblies includesixteen fuel rods having a concentration of gadolinium in a range of 5%to 12% by weight and four fuel rods having a concentration of gadoliniumin a range of 0.5 to 2% by weight, the other fuel rods being devoid ofgadolinium.